EMPLACEMENT MECHANISMS OF THE SYNTECTONIC MCCARTNEY PLUTON IN THE SW MONTANA FOLD-THRUST BELT: STRUCTURAL, MICROSTRUCTURAL, AND AMS STUDIES

The Late Cretaceous McCartney Mountain pluton, located 30 km N of Dillon, is a satellite of the Eastern Pioneer batholith exposed ~10-15 km to the W. This small pluton (4-5 km diameter) intruded the SW Montana fold-thrust belt syntectonically. The magmatic fabric of the pluton has been evaluated using a combination of field measurements, image analysis, and anisotropy of magnetic susceptibility produced by magnetite, the main magnetic carrier in this pluton. Foliation shows an approximately concentric pattern, describing the flat lens shape of the pluton, which has a northern tilt. The pluton appears to root in the western region, toward the Pioneer batholith, consistent with the observed gravity and magnetic anomalies. AMS lineation is down-dip in general, except along the western margin where it is subhorizontal.

Structures in the country rocks are markedly different between the W and E contacts of the pluton. The W region, between the Pioneer batholith and the pluton, underwent metamorphism and ductile deformation while the E region has preserved the shallow-level deformation of the fold-thrust belt. In the W, ductilely deformed rocks prevail for about 3 km from the pluton contact, showing tightly folded quartzite, slate, calc-silicate and marble of the Cretaceous Colorado Group. Stretch clasts in quartzite metaconglomerates indicate a horizontal stretch S=2-3. Quartz c-axes fabrics in the quartzite layers and calcite microfabrics in marble indicate that deformation-recrystallization took place during bulk coaxial flow with a small component of simple shearing.

We propose that the McCartney Mountain pluton intruded during the development of the fold-thrust belt and was fed by conduits from the Pioneer batholith. The pluton grew in part by crack propagation in the Colorado Group to the east and by ductile deformation of the western contact that had been thermally softened. Subhorizontal lineations in both the pluton and the western contact zone are attributed to a process akin to the growth of a horizontal lens; maximum stretch occurs along the lens horizontal circumference. Magmatic and solid-state structures within and around the McCartney Mountain pluton indicate that this pluton grew in a hybrid manner: brittle processes along the eastern margin and ductile strain in the thermally softened western edge. [We thank the UMN 2002 Adv Field Camp class]